A typical electric power cable generally comprises one or more conductors, which form a cable core that is surrounded by several layers of polymeric material including a first semiconducting shield layer, an insulating layer, a second semiconducting shield layer, a metallic tape or wire shield, and a jacket.
These insulated cables are known to suffer from shortened life when installed in an environment where the insulation is exposed to water, e.g., underground or locations of high humidity. The shortened life has been attributed to the formation of water trees, which occur when an organic polymeric material is subjected to an electrical field over a long period of time in the presence of water in liquid or vapor form. The net result is a reduction in the dielectric properties of the insulation.
Many solutions have been proposed for increasing the resistance of organic insulating materials to degradation by water treeing. The most recent solution involves the addition of polyethylene glycol (PEG), as a water tree growth inhibitor, to a heterogeneous low density polyethylene such as described in U.S. Pat. Nos. 4,305,849; 4,612,139; and 4,812,505. The addition of PEG to polyethylene, however, presents certain problems, particularly in the areas of process and long term heat stability and in compatibility with the host polymer. The latter is addressed by selecting a PEG of a particular molecular weight (weight average molecular weight); however, compounding conditions may still produce a low molecular weight fraction. The former requires the addition of a high level of certain heat stabilizers, which cause staining of the composition. The color (or stain) produced and the reduction in crosslinking, both due to the high level of heat stabilizer, lead to some commercial difficulties. Thus, there is an industrial demand for water tree retardant additives that are as effective as PEG, but do not present stability and compatibility concerns.